The present invention relates to electric motor driven submersible pumping systems and, more particularly, to a method and apparatus for controlling operation of a submersible pump or other variable torque load.
Deep well, residential service, submersible pumps presently available in the market are driven with two pole, alternating current (AC) induction motors which have been packaged to survive immersion in the well. The stator portion of the motor is encapsulated with an epoxy making it impervious to moisture and the encapsulated motor is enclosed in a housing assembly with water lubricated bearings. The enclosure is then filled with ethylene-glycol. The motor output shaft is directly coupled to a shaft of a pump which has a stack of impellers to force water into an outlet pipe. The outlet pipe has a pressure level determined by the depth of the well and the pressure level at the associated residence. A check valve is included in the pump outlet pipe to prevent water draining into the well when the pump outlet pressure is less than the pressure in the outlet pipe.
The most popular motor rating for residential water pumping systems is 3/4 horsepower with a 1.6 service factor having a net continuous rating of 1.2 hp. The motor and pump are coupled in line and typically fit into an outer casing 4" in diameter with a total length of the assembly of about three to four feet. The pump/motor assembly with wiring and supply pipe attached is lowered into the well to a depth that keeps it a short distance from the bottom to avoid sand and other contaminants from fouling the water inlet. Maximum operating depth can be up to 400 feet and the pump capacity is preferably sufficient to maintain 60 psi plus the pressure needed to overcome the 400 foot head.
The pumping system at the top of the well includes a storage tank with a spring loaded or air initiated bladder to minimize the change in pressure with dropping water level in the tank as water is used by the residence. A pressure switch with adjustable hysteresis is interfaced to the storage tank to switch the pump "ON" when the pressure drops below a minimum set point and "OFF" when the pressure reaches a maximum set point.
There are several deficiencies with the present commercially available design including, for example, the following: (1) a four inch pump-motor diameter requires a five inch well casing at a substantial well drilling cost; (2) if a well is pumped dry, the pump may be damaged because the bearings are water lubricated and the lack of water leads to bearing failure unless a flow restrictor is added to the waterline at the well head to prevent the output flow from exceeding the well recovery rate; (3) sand, stone chips or other debris in the well may cause the pump to seize or bind leading to a stalled motor condition that may cause motor overheating and damage; (4) if line voltage is low, the motor is forced to operate at less than rated magnetic flux, thus requiring more current to produce the same torque leading to motor overheating and the possibility of eventual failure; (5) starting the motor by connecting it across the AC power line results in a significant surge in input current with each start and frequent restarts, such as those experienced when the power is frequently interrupted during thunderstorms, can also lead to motor overheating and failure; and (6) because of the heating penalty associated with each start, the hysteresis in the pressure switch must be increased to a value great enough to ensure that the motor doesn't restart too frequently forcing the homeowner to endure pressure variations of as much as 50% between pump starts.
An AC induction motor typically has a pullout torque (maximum torque on the motor characteristic curve) which is 3 to 4 times the rated torque and a current at stall which is 5 to 6 times the rated current. In an application where the motor is started by simply connecting it across the power source using a switch or contactor, there is an initial inrush current of 5 to 6 times the rated current which gradually reduces to rated current as the motor accelerates to rated speed. During the acceleration, the torque increases with increased speed until the pullout torque speed is reached, after which the torque and current begin to fall as the speed increases further. The speed will settle to a constant value when the motor torque is equal to the load torque.
Torque loads presented to the motor by pumps and other variable speed loads, such as compressors and fans, vary with shaft speed. With these types of loads, the load torque at zero speed is very small and increases with increasing speed. The torque available to accelerate the load is the difference between the motor torque and the load torque. The ideal fan torque characteristic is a torque which varies with the square of speed. Pumps and compressors are oftentimes similar to the fan load torque, but in some instances may depart significantly from the ideal characteristics due to variations in back pressure, for example. In general, torque can be considered to be a function of slip frequency where a linear approximation has sufficient accuracy for most applications. If motor speed is known from a tachometer or other speed measuring device, then all a controller must do in order to produce a desired level of torque at that speed is to calculate what frequency would place the synchronous speed at the rotor speed, and then add to that the slip frequency needed to produce the desired torque. For example, if the motor is running at 1800 rpm, 30 Hz excitation would make this the synchronous speed for a two pole motor. Typically, a slip frequency of 3 Hz provides 200% of rated torque so that providing 33 Hz excitation at this speed will result in 200% torque. This principle of control is usually referred to as slip control and is well known in the art.
In highly competitive markets, a tachometer or other speed sensor adds too much cost to a controller, and systems are built without speed sensing. Without speed sensing, the speed must be changed slowly to ensure that the motor continues to operate at slip frequencies equal to or less than the frequency corresponding to pullout torque. The reasoning for this is that when the frequency source is an electronic unit where the maximum current determines the controller cost, maximum current limit is usually set at about twice the required continuous current rating by cost constraints. If the frequency is allowed to increase significantly faster than the motor speed, the system may get into a state where the slip frequency is so high that the current limit causes the maximum torque developed to be significantly less than rated torque causing the motor to stall. If there is no speed measuring device, there may be no way for the controller to recognize that a stall has occurred and current will continue to be supplied at the limit value causing the motor to overheat and be damaged. While the description of this concern was based upon increasing the frequency too fast, the same state may arise as the result of load torque impulses, sticky shafts, and other anomalies that cause the motor shaft speed to drop.